1. Field of the Invention
The present invention relates generally to an electrophotographic apparatus and method for use in a printer, a facsimile, etc., in which dot latent images in conformity with binarized image dot data are formed on a photosensitive drum through scanning of an LED array or a laser beam and in which after development of the latent images with toners the resultant dots are transferred onto record paper, and more particularly to an electrophotographic apparatus and method providing control of appropriate quantity of light emission taking into consideration the influence of light from lighting peripheral dots.
2. Description of the Related Arts
In the case of conventional electrophotographic apparatuses used for image printing in laser printers, facsimiles or the like, record paper is conveyed at a certain speed by a paper conveyance mechanism so that images are recorded on the record paper through the electrophotographic process executed by electrostatic recording units arranged in the record paper conveyance direction. The electrostatic recording unit forms latent images in conformity with image dot data by means of LED array line scanning or laser diode laser beam scanning on a photosensitive drum in rotation, and after development with toner components, transfers the developed images onto record paper. In recent years, color electrophotographic apparatuses for recording color images have also been put into practice. The color electrophotographic apparatus comprises four electrostatic recording units of black (K), cyan (C), magenta (M) and yellow (Y) in a tandem manner in the record paper conveyance direction. The electrostatic recording units for four colors optically scan the photosensitive drums on the basis of image data to form latent images, develop the latent images with color toners in developing vessels, transfer the developed images onto record paper being conveyed at a certain speed in a superposed manner in the order of yellow (Y), magenta (M), cyan (C) and black (K), and finally perform thermal fixing through a fixing device or other processes.
However, such conventional electrophotographic apparatuses have entailed the following problems since the LED array or the laser diode issues a uniform optical energy per dot to perform development and recording. Now assume that the optical energy has been determined so as to be able to develop isolated one dot on white background. FIG. 1 shows a relationship between a latent image and a bias voltage in the case where the photosensitive drum has been subjected to optical energy capable of developing an isolated one dot in the electrophotographic process. A latent image 200 is not developed as a toner image at a specific bias voltage Vth or below, but it is developed as a toner image 202 at a voltage exceeding the bias voltage Vth, with the dot size of W1. However, in the case where the optical energy equal to FIG. 1 is supplied to the adjoining two dot positions to develop two dot lines as shown in FIG. 2, latent images 204 and 206 indicated by broken lines are formed for respective dots, resulting in a combined latent image 208 indicated by solid line in its entirety, whereupon a toner image 210 is developed for broken line latent images exceeding the bias voltage Vth. Such a combination of the optical energy supplied to the adjoining dots leads to the dot size W2 exceeding the original two dots. Alternatively, in cases where the optical energy equal to FIG. 1 has been supplied to two dots with one dot space as shown in FIG. 3, combination of latent images 212 and 214 indicated by broken lines result in a latent image 216 indicated by a solid line, whereupon a toner image 218 comes to have three dot size W3 with the crushed intermediate dot. In case of presence of another dot around the lighting dots in this manner, mutual influence will cause a crush of the isolated white point when the high-density pattern is printed, with another problem in that halftone pattern of the order of 50% will bring about saturation of density, leading to the density of solid print area.
On the contrary, if the optical energy per dot is determined as shown in FIG. 4 so as to prevent the density of the high-density part from saturating, the latent image 220 may not reach the bias voltage Vth due to too small optical energy when the one dot isolated point is printed, which makes it impossible to develop the dots. For this reason, respective drawbacks lie in both the method of determining the optical energy so as to enable isolated one dot to be developed and the method of determining the optical energy so as to prevent the density of a high-density part from saturating, with the mutual trade-off relationship. Therefore, in reality, intermediate value of the two methods is employed with respective some problems unsolved.
Further, the drawbacks that the emission of light of a plurality of adjoining dots may cause too a large dot size and that no isolated dot may be formed by the high-density optical energy, will be attributable to the non-linear relationship as shown in FIG. 5, in which the relation between the developed dot diameter and the optical energy is not linear. That is, the dots are not formed with an optical energy L short of a certain threshold value Lth. Once the optical energy exceeds the threshold value Lth, the dot diameter W sharply increases in accordance with the increase of the optical energy P. Afterwards, increase of the dot diameter W become gradually gentle with respect to the increase of the optical energy P.